In the manufacture of flat-rolled stainless sheets, it is necessary to anneal or soften the material for further cold-rolling operations. It is also necessary to anneal the material at the finish gauge to render it suitable for fabrication by stamping, forming, and the like. Annealing is necessary because cold rolling elongates the grains of the stainless steel, greatly distorts the crystal lattice, and induces heavy internal stresses. The steel that results from the cold rolling process is typically very hard and has little ductility. The annealing process allows the cold-worked steel to recrystallize, and if the steel is held at the proper annealing temperature for a sufficient time, the structure of the annealed steel will again consist of undistorted lattices and the steel will again be soft and ductile.
The continuous annealing process involves unwinding the coil from a payoff reel and continuously feeding the coil into and pulling the coil through a furnace and then rewinding the coil on a take-up reel. The furnace is typically electric or gas fired. The steel strip, while traveling in the furnace, is typically heated to a temperature in the range of about 1000° C. to about 1200° C. in the case of austenitic alloys and to a temperature in the range of about 750° C. to about 1000° C. for ferritic alloys. The annealing temperatures vary depending upon the particular alloy being annealed, as well as the intended end-use of the alloy.
A bright-annealing plant basically comprises a bright-annealing furnace, preceded by a degreasing unit. The furnace is operated with a hydrogen atmosphere so that the stainless steel strip does not oxidize at the high annealing temperatures. There are, however, trace amounts of moisture and oxygen in the hydrogen atmosphere of the furnace. Depending on the level of the dew point in the furnace, oxide layers can form on the stainless steel strip. In particular, during start-up of the furnace there may be higher concentrations of oxygen, causing such extensive oxide formation that the oxides become visible on the surface of the stainless steel in the form of annealing colors. In order to eliminate these problems, many bright-annealing plants are equipped with an electro-chemical nitric acid treatment stage in the discharge section of the annealing furnace. It is not possible, however, to quantify exactly the success of this treatment. Sometimes the stainless steel strip shows even stronger annealing colors after the nitric acid electro-chemical treatment. In fact, most new bright-annealing plants are planned and built without this nitric acid electro-chemical post-treatment stage. This is primarily due to the fact that the annealing furnace atmosphere can be controlled much more effectively and the dew point no longer fluctuates to the same extent as before.
The advantage of a bright-annealing plant is that the surface qualities that can be obtained provide very high brightness and very low roughness levels. Compared with a cold-rolled stainless steel strip from a conventional plant which is annealed and pickled, the stainless steel material produced from the bright-annealing plant has the disadvantage of having a different surface composition. The surface composition of the surface scale is a disadvantage in the subsequent mechanical processing stages, such as tension-leveling, skin-pass rolling, punching, and deep-drawing.
The surface scale of the bright annealed stainless steel can often cause considerable difficulties if it is run through the stretcher-and-roller leveling machine that is used to improve flatness. Surface material from the strip can be deposited and form a coating on the rollers. In addition, micro-cracks can form in the strip surface which have an adverse effect on the brightness of the stainless steel. The hard particles on the surface of the bright annealed material lead to much shorter work roller service life in the skin-pass mill.
The surface scale on the bright annealed stainless steel also result in shorter tool service life of cutting and deep-drawing machinery. In addition, the modern cutting presses which operate at up to 1000 cuts per minute often cannot achieve full productivity with the bright annealed material. In the deep-drawing process the precision is often lacking if the material has different friction coefficients on the surface.
In recent years, improvements in bright annealing plants are the result of the introduction of 100% hydrogen atmosphere, high convection devices, improved furnace design, and modern computer controls. These improvements in the batch annealing technology have resulted in an increase of energy efficiency and improvement of heat transfer rates during both heating and cooling periods, thereby producing more uniform properties throughout the coil and reducing the process cycle time by more than 50% over older batch annealing operations. These improvements, together with alternative impeller materials, have resulted in maximum temperatures attainable in commercially available annealing furnaces of approximately 900° C. or more.
Accordingly, there is a continuing need in the industry for an improved process for treating stainless steel.